Methods and compositions for the selective etching of silicon

ABSTRACT

Methods and compositions for the selective etching of silicon in the presence of p-doped silicon are disclosed whereby a portion of a silicon surface may be dissolved while a p-doped pattern in the surface remains substantially undissolved. The compositions comprise (a) an aqueous solution of an alkali metal hydroxide or a tetraalkylammonium hydroxide; and (b) a high flash point alcohol, phenol, polymeric alcohol, or polymeric phenol.

TECHNICAL FIELD

The invention relates to methods and compositions for the selectiveetching of silicon in the presence of p-doped silicon whereby a portionof a silicon surface may be dissolved while a p-doped pattern in thesurface remains substantially undissolved. The compositions comprise (a)an aqueous solution of an alkali metal hydroxide or a tetraalkylammoniumhydroxide; and (b) a high flash point alcohol, phenol, polymericalcohol, or polymeric phenol.

BACKGROUND ART

The manufacture of semiconductor integrated circuits typically involveshighly complex, time consuming and costly processes which, withcontinually narrower line width requirements, must be achieved with anever increasing degree of precision. Within such processes the etchingof semiconductor material (e.g., silicon) often entails the use of achemical bath to which a patterned semiconductor material is exposed, soas to etch selectively certain portions of the surface of a wafer. In atypical chemical etch process, both the rate of etch and the selectivityof etch are parameters critical to the successful formation of anintended substrate geometry. For the fabrication of devices having aso-called surface "strap", it is desirable that an etchant be able toselect between a silicon substrate and a p-doped pattern in the silicon.Thus, for fabricating a semiconductor device having a p-doped strap on asilicon surface, the silicon surface can be implanted or diffused with ap-type dopant, such as boron, in a pattern containing at least 2×10¹⁹atoms/cm³ of boron in the appropriate configuration. If one then has anetchant that can distinguish between the p-doped silicon and the undopedsilicon, one can dissolve a portion of the surface of the silicon andleave behind a raised pattern of p-doped straps.

It is known in the art to employ potassium hydroxide(KOH)-based etchantsfor anisotropically etching silicon, and various lower alkyl alcoholshave been added to the KOH solution to control the etching process.Thus, Kragness et al. (U.S. Pat. No 3,506,509) describes an etchantcomprising KOH, n-propanol and sec-butanol which is said to exhibit asubstantially level etch rate. Erdman and Schmidt (U.S. Pat. No.3,738,881) disclose an etchant comprising potassium, sodium, cesium,rubidium or quaternary ammonium hydroxide and an aliphatic alcohol. Theyindicate that aliphatic alcohols, such as n-propyl, isopropyl, n-butyl,sec-butyl, tert-butyl, pentyl and hexyl alcohols, are more suitablepassivating agents for their purposes than are aromatic alcohols. Otherpatents disclose etchants comprising (a) alkali hydroxides and (b)methanol, ethanol, n-propanol, ethylene glycol, ethylene diamine, andother amines. Hall et al. (U.S. Pat. No. 3,160,539) disclose a solutionof catechol in hydrazine as a silicon-selective etchant, although thesignificant hazards of hydrazine and the rapid oxidative deteriorationof catechol make it industrially unattractive. Our own U.S. Pat. No.4,941,941 discloses an etching solution comprising an aromatic compoundhaving at least two adjacent hydroxyl groups, an amine and water. Twogood reviews of anisotropic etching have been published: E. BassousElectrochemical Technology in Electronics ECS Symposium 1988, p. 123 andW. Kern RCA Review 39, 278-308 (1978) p. 292.

Of the known methods for controlled etching, the combination of KOH andisopropanol is probably the most commonly used at present. Like all theother etchant compositions of the art, it suffers from two majordrawbacks: (1) large proportions of the alcohol component are necessaryto achieve modest selectivity between doped and undoped silicon, and (2)the alcohol must be used in an industrial setting at temperatures aboveits flash point.

Thus, there is a need for an etchant composition and method that do notgive rise to the safety hazards associated with using flammable solventsabove their flash points.

There is a further need for a composition and method that are highlyselective for silicon in the presence of p-doped silicon.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an etchantcomposition that is highly selective for silicon in the presence ofp-doped silicon.

It is a further object to provide an etchant composition that can beused well below the flash point of any component.

These and other objects and features are realized in the presentinvention, which relates to a method for the controlled etching of asilicon substrate comprising exposing the silicon substrate to a mixtureof (a) an aqueous solution of a base chosen from the group consisting ofalkali metal hydroxides and tetraalkylammonium hydroxides and (b) analcohol of structure I: ##STR1## wherein Ar is aryl or substituted aryl;

R' is hydrogen or linear lower alkyl;

n is zero or an integer from 1 to 4; and

m is zero or one.

When Ar is substituted aryl, the aryl is preferably substituted withlower alkyl or halogen. The term lower alkyl as used herein refers tohydrocarbon radicals of four or fewer carbon atoms.

Preferred alcohols are benzyl alcohol, α-methylbenzyl alcohol,phenoxyethanol and phenylpropanol and they are preferably present in anamount from 0.1% by weight to a saturated solution. Preferred bases arepotassium hydroxide, sodium hydroxide or tetramethylammonium hydroxideand they are preferably at a concentration from 0.5 to 3.0N. Potassiumhydroxide is most preferred.

In another aspect, the invention relates to a method for the controlledetching of a silicon substrate comprising exposing the silicon substrateto a mixture of (a) an aqueous solution of a base chosen from the groupconsisting of alkali metal hydroxides and tetraalkylammonium hydroxidesand (b) a phenol or substituted phenol having one or more substituentschosen independently from the group consisting of alkyl, halogen, alkoxyand nitro. Preferably the phenol has the structure II: ##STR2## whereinR₂ is hydrogen or lower alkyl; and R³ is hydrogen or lower alkyl.

Preferably the aqueous solution is from 0.5 to 3.0 N KOH and the phenolis a cresol.

In another aspect the invention relates to a method for the controlledetching of a silicon substrate comprising exposing the silicon substrateto a mixture of (a) an aqueous solution of a base chosen from the groupconsisting of alkali metal hydroxides and tetraalkylammonium hydroxidesand (b) a polymer, the repeating units of which possess hydroxylfunctions. Preferred polymers are novolac resins, poly(hydroxystyrenes)and poly(vinyl alcohols). A preferred novolac has the formula III:##STR3## wherein p is from 20 to 85. In preferred embodiments, theaqueous solution is from 0.5 to 3.0 N KOH, the novolac is present in anamount from 0.05 ppm to 2% by weight and the silicon substrate isexposed to the mixture at about 45° to 105° C.

Preferred poly(hydroxystyrene)s have the formula IV: ##STR4## wherein qis from 20 to 200. The preferred styrenes are present in an amount from0.05 ppm to 2% by weight. When the polymer is poly(vinyl alcohol), apreferred aqueous solution is from 0.5 to 3.0N KOH, the poly(vinylalcohol) is of molecular weight from 5,000 to 10,000 and it is presentin an amount from 0.01% to 5% by weight.

In another aspect, the invention relates to a method for fabricating athree-dimensional p-doped structure on a surface of a silicon wafercomprising:

(a) patterning the surface of the wafer with a pattern containing atleast 2×10¹⁹ atoms/cm³ of p-dopant; and

(b) exposing the surface to a solution of a polymer, an alcohol or aphenol and an aqueous base, said polymer having repeating units whichpossess hydroxyl functions, and said base chosen from the groupconsisting of alkali metal hydroxides and tetraalkylammonium hydroxides,whereby a portion of the silicon surface of the wafer is dissolved andthe p-doped pattern remains substantially undissolved. When component(b) is an alcohol, the alcohol has the formula I. When component (b) isa phenol, the phenol has the formula II. When component (b) is apolymer, it is preferably a novolac of formula III, apoly(hydroxystyrene) of formula IV or a polyvinyl alcohol.

In a closely related aspect the invention relates to a method foretching a surface of a silicon substrate, said surface having implantedor diffused therein a boron-doped pattern with a dopant level above2×10¹⁹ atoms/cm³. The method comprises exposing the surface to asolution of from 0.05 to 300 ppm of novolac resin in from 0.5 to 2.0Naqueous potassium hydroxide at 45° to 105° C. whereby a portion of thesilicon substrate is dissolved and the boron-doped pattern remainssubstantially undissolved.

In a composition aspect, the invention relates to a composition for thecontrolled etching of silicon consisting essentially of:

(a) an aqueous solution of a base at a concentration from 0.5 to 3.0N,said base chosen from the group consisting of alkali metal hydroxidesand tetraalkylammonium hydroxides; and

(b) from 0.001% to 5% by weight of an alcohol of formula I. Thecomposition may additionally contain a surfactant. In a preferredcomposition the base is potassium hydroxide and the alcohol is chosenfrom the group consisting of benzyl alcohol, α-methylbenzyl alcohol,phenoxyethanol and phenylpropanol.

In other composition aspects, the invention relates to similarcompositions in which the alcohol of formula I is replaced by a polymer,the repeating units of which possess hydroxyl functions. Preferably, thepolymer is a novolac resin of molecular weight 2,500 to 10,000. Thecompositions may also contain a surfactant the alcohol and polymer canbe mixed.

BRIEF DESCRIPTION OF DRAWINGS

FIGURE 1 is a plot of etch ratio vs boron doping concentration for asolution of the art and three solutions of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the method of the invention, a silicon substrate having ap-doped pattern may be selectively etched so that the silicon isdissolved and the p-doped pattern remains substantially undissolved. Theetch ratio between undoped and doped is a measure of the utility of theprocess: the higher the better. In addition, and relatedly, the lowerthe level of dopant required for functional differentiation, the better.That is, if one can reduce the level of p-dopant that must be implantedor diffused into the silicon to form the pattern and still have theetchant be able to select silicon over doped silicon, the fabrication ismade simpler and less expensive.

It has been found that significant improvement over the compositions andmethods of the art can be obtained, both in terms of selectivity and interms of safety, by utilizing mixtures of typical bases known and usedfor etching silicon together with higher molecular weighthydroxyl-containing etch moderators.

FIGURE 1 illustrates the higher selectivity of the compositions of theinvention. Curve A represents the etch ratio of a 2N KOH/25% isopropylalcohol etchant of the art at 45° C. as a function of boron dopantconcentration in atoms/cm³. Curves B, C and D are compositions of theinvention. B is 2.5% benzyl alcohol in 2N KOH at 45° C.; C is 2.5%benzyl alcohol in 2N KOH at 60° C.; and D is 2.0% benzyl alcohol in 2NKOH at 45° C.

Typical bases include alkali metal hydroxides, such as sodium,potassium, and, less commonly, cesium and rubidium hydroxides. Anotherclass of basic etchant often used when alkali metal ions would interferewith subsequent processing, is the "metal-ion-free" or "MIF" class.These are primarily tetraalkyl ammonium hydroxides, the most commonlyused MIF being tetramethylammonium hydroxide (TMAH). When used alone,the basic etchants above show no useful selectivity between doped andundoped substrates.

Although the inventor does not wish to be bound by any theoreticalexplanation for the results, it is possible that the aralkyl alcohol andpolymeric alcoholic moderators interact with existing or nascent Si-OHgroups on the surface of the silicon, thereby restricting access to thesurface by the base. The advantage to the polymers arises from theirability to complex extensively with the surface. This may be reflectedin the observation that relatively small amounts of the polymers arevery effective. In any event, among aralkyl alcohols, benzyl alcohol,α-methylbenzyl alcohol, phenoxyethanol and phenylpropanol seemparticularly well suited to the method of the invention. Among themembers of the genus of structure I, 2-phenylethanol has also beentested but is less selective, which may be a function of its lowersolubility in the aqueous etchant solution. Monomeric phenols, depictedby formula II, tend to be soluble to a greater extent than aralkylalcohols and are very effective moderators. However, mammalian toxicitymakes some of the phenols less attractive in an industrial process.Polyhydroxyphenols, particularly catechols and hydroquinones, and to alesser extent, resorcinols, tend to oxidize and are therefore notpreferred for the inventive etchants. In this regard, it has been foundthat the results of processes that employ benzyl alcohol at elevatedtemperatures are improved by restricting the opportunities for itsoxidation, for example, by blanketing the solution with an inert gas.The hydroxyl-containing moderators are useful up to the limit of theirsolubility in the aqueous base. This will, of course, be a function ofthe temperature at which the etch is performed.

In the process of the invention, the etch can be carried out at anytemperature from ambient to the boiling point of the solution. Forpractical purposes the etch is preferably carried out above 45° C., and,as will be seen below, the selectivity of some of the moderatorsimproves as the temperature increases. The boiling point will depend tosome extent on the concentration of base, 3.0N solutions having aboiling point a few degrees higher than 100° C.

The etchant solutions may be used for fabricating three-dimensionalp-doped structures on the surface of silicon wafers. For example, awafer is implanted with a pattern by ion implantation or by diffusionthrough a pattern in a mask; a layer of polysilicon is deposited on thesurface of the wafer; and the wafer is heated to diffuse the dopant,usually boron, into the polysilicon in a pattern reflecting theoriginal. The polysilicon surface of the wafer is then exposed to theetchant solutions of the invention, and the undoped polysilicon isdissolved away, leaving the p-doped polysilicon behind as a raisedpattern on the surface of the wafer. In this process, the less boronthat must be implanted or the less strenuous heating conditions (timeand temperature) to diffuse the pattern, the better. Thus, selectivityof the etchant is a major determinant of the economics of the process,and the etchants of the invention are a significant advance over theart.

Several experiments were undertaken to demonstrate the improvedselectivity of the solutions of the invention. (The safety improvementsare readily shown from an examination of the flash points of thecompounds of the inventive solutions, which are available in referencesof the art.)

Each of the etchants found in the Results section was prepared fromstock solutions which were approximately 2N KOH. Solutions of alcoholswere formulated by adding the appropriate amount of alcohol to 100 mL of2N KOH stock solution and, if incompletely dissolved, rolling on aroller mill overnight. This procedure produced solutions saturated atroom temperature. The undissolved alcohol was removed using a separatoryfunnel. Samples of boron-doped (3 to 6×10¹⁹ atoms/cm³) or intrinsicpolysilicon films were deposited on silicon wafers. These films wereetched at a given temperature, and the rate of dissolution wascalculated. Conditions for the individual runs appear in Table I. Aone-minute 10:1 buffered oxide etch clean step preceded the alcoholicetch step.

The rate of dissolution of intrinsic polysilicon, of boron-dopedpolysilicon and the ratio of the two rates are shown in Table I. Bycomparison, the existing standard technology, 25% isopropyl alcohol at45° C. (flash point =22° C.) in 2N KOH exhibits an etch ratio of about6. The term "substantially undissolved" as used herein thus refers todifferential dissolution where the etch ratio is about 6 or more.

                                      TABLE I                                     __________________________________________________________________________                          Rate I  Rate B                                          Moderator  Temp.                                                                              Conc. (Å/min)                                                                           (Å/min)                                                                         Ratio                                     __________________________________________________________________________    PhCH.sub.2 OH                                                                            45   2%    .sup. 149.sup.a                                                                       45    6.0*                                                 45   2%    946     172   5.5*                                                 45   2%    400     16.8  23.8**                                               45   2.2%  828     142   5.8*                                                 45   1.8%  1121    176   6.4*                                      PhCH(CH.sub.3)OH                                                                         45   sat'd 143     8.2   17.2*                                                45   1.5%  158     32    4.9*                                                 45   0.45% 271     18    15                                                   55   0.50% 285     53    5.4                                       PhOCH.sub.2 CH.sub.2 OH                                                                  45   sat'd 267     38    7.1*                                      PhCH.sub.2 CH.sub.2 OH                                                                   45   sat'd 127     37.2  3.4*                                      PHCH.sub.2 CH.sub.2 CH.sub.2 OH                                                          45   sat'd 190     30.8  6.1*                                      o-cresol   45   9%     32     5.8   5.6*                                                 45   1%     62     8.8   7.1                                                  45   2%     44     7.5   5.9                                                  45   3%     56     10.5  5.3                                                  45   4%      40.1  12.5  3.2                                       m-cresol   45   9%    123     10.5  11.7*                                     novolac    45   1%     75     6.3   12                                                   45   2%     82     8.2   10                                                   45   3%     72     8.3   8.8                                                  45   4%      80.6  7.6   10.6                                                 55   4%    139     13.1  10.7                                                 65   4%      317.5 22.5  14.1                                                 50   2%     59     6.6   9.0                                                  55   2%    103     5.8   17.8                                                 60   2%    165     8.6   19.3                                                 65   2%    241     10.7  22.5                                                 70   2%    317     13.3  23.8                                                 50   1%     75     6.1   12.3                                                 55   1%    116     8.2   14.1                                                 60   1%    154     5     30.8                                                 65   1%    245     8.2   29.9                                                 70   1%    366     11.3  32                                                   104  3%    2.26 × 10.sup.3                                                                 --    --                                                   104  0.1%  3.55 × 10.sup.3                                                                 --    --                                                   104  66.6 ppm                                                                            5.12 × 10.sup.3                                                                 --    --                                                   104   6.6 ppm                                                                            9.83 × 10.sup.3                                                                 --    --                                                   104   0.6 ppm                                                                            2.01 × 10.sup.4                                                                 --    --                                                   104   0.3 ppm                                                                             3.3 × 10.sup.4                                                                 --    --                                                   104  0.06 ppm                                                                            3.33 × 10.sup.4                                                                 --    --                                                   104  0     3.33 × 10.sup.4                                                                 --    --                                        poly(vinyl alcohol)                                                                      104  1.5%   5.8 × 102.sup.2                                                                --    --                                        mw .sup.˜  5000                                                                    104  1.0%   5.5 × 10.sup.2                                                                 --    --                                                   104  0.3%   7.0 × 10.sup.3                                                                 --    --                                                   104  0.1%   1.1 × 10.sup.4                                                                 --    --                                                   104  0.05%  1.7 × 10.sup.4                                                                 --    --                                        __________________________________________________________________________     *The values marked were obtained under nonequilibrium conditions (no          stirring). Unmarked values were obtained with stirred solutions.              **This value was obtained on production runs in the appropriate equipment     .sup.a The difference in absolute dissolution rates between two 2% benzyl     alcohol samples is believed due to variations in the KOH concentration.       The ratio is unaffected.                                                 

An additional feature of the use of low concentrations of novolac as themoderator was the observation that at low KOH concentration (0.5 to1.0N) and less than 0.1 ppm novolac, the silicon surface that emergedafter etching and rinsing appeared unusually smooth. For certainapplications there may be an additional advantage to this apparentimprovement in etch uniformity.

The alcohols of formula I, the phenols of formula II, and the polymersof formulas III and IV as well as the poly(vinylalcohols) may beprepared by procedures well known in the art. Most are commerciallyavailable. The novolac resins, which are available from Shipley (Newton,Mass.) and Kodak (Rochester, N.Y.), are particularly attractivecandidates as moderators because they do not exhibit measurable flashpoints and, in the quantities needed, they are inexpensive to purchaseand to dispose of properly after use. Novolac resins arephenol-formaldehyde resins; a procedure for their manufacture isdescribed in Chemistry and Application of Phenolic Resins, Knop andScheib, Springer Verlag N.Y. 1979.

Because surfactants are occasionally included in etch solutions, apotassium perfluorocarboxylate surfactant (FC-129, 3M, St. Paul, Minn.)was included in one experiment with 0.5% α-methylbenzyl alcohol; theetch ratio was reduced from 15 to 8.4. Apparently this particularsurfactant interferes to some extent with the protection afforded byα-methylbenzyl alcohol. Nonetheless, the solution still exhibits verygood selectivity and the inclusion of surfactants may, in someinstances, provide advantages that offset their diminution of the etchratio. Examples of surfactants that are commonly used in etchantsolutions include Fluorad™ (3M-St. Paul) and Dowfax (Dow, Midland,Mich.).

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that other changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

We claim:
 1. A method for the controlled etching of a silicon substratecomprising exposing said silicon substrate to a mixture of (a) anaqueous solution of a base chosen from the group consisting of alkalimetal hydroxides and tetraalkylammonium hydroxides and (b) an alcohol ofstructure ##STR5## wherein Ar is aryl or substituted aryl; R' ishydrogen or linear lower alkyl;n is zero or an integer from 1 to 4; andm is zero or one.
 2. A method according to claim 1 wherein Ar is aryl oraryl substituted with alkyl of 1 to 4 carbons or halogen.
 3. A methodaccording to claim 1 wherein said alcohol is present in an amount from0.1% by weight to a saturated solution.
 4. A method according to claim 3wherein said aqueous base is potassium hydroxide, sodium hydroxide ortetramethylammonium hydroxide.
 5. A method according to claim 4 whereinsaid aqueous solution of a base is from 0.5 to 3.0N potassium hydroxide.6. A method according to claim 3 wherein said alcohol is chosen from thegroup consisting of benzyl alcohol, α-methylbenzyl alcohol,phenoxyethanol and phenylpropanol.
 7. A method for fabricating a p-dopedstructure on a surface of a silicon wafer comprising:(a) patterning saidsurface of said wafer with a pattern containing at least 2×10¹⁹atoms/cm³ of p-dopant; and (b) exposing said surface to a solution of analcohol and an aqueous base, said alcohol having the formula ##STR6##wherein Ar is aryl or aryl substituted with lower alkyl or halogen; R'is hydrogen or linear lower alkyl; n is zero or an integer from 1 to 4;and m is zero or one;and said base is chosen from the group consistingof alkali metal hydroxides and tetraalkylammonium hydroxides, whereby aportion of said silicon surface of said wafer is dissolved at a rate atleast 6 times that of said pattern.